The present invention relates to a switching power supply apparatus for supplying a stabilized DC voltage to industrial and consumer electronic apparatuses. More particularly, the present invention relates to an overcurrent protection circuit of a switching power supply apparatus. The overcurrent protection circuit prevents overcurrent flowing through the switching power supply apparatus itself and apparatuses connected to the input and output of the switching power supply apparatus in an overload condition.
In recent years, electronic apparatuses are made more inexpensive, compact, efficient and energy saving. Accordingly, switching power supply apparatuses for these electronic apparatuses are strongly demanded to have higher output stability and to be more compact and efficient. At the same time, switching power supply apparatuses being high in safety are demanded in the field of electronic apparatuses. Even when an electronic circuit serving as a load causes an abnormality and its input impedance lowers, an overcurrent protection circuit of a switching power supply apparatus satisfying the above-mentioned demands is required to appropriately restrict the current flowing through the electronic circuit serving as the load and to maintain the electronic circuit in a safe condition.
A conventional overcurrent protection circuit of a switching power supply apparatus will be described below referring to an accompanying drawing, FIG. 12. FIG. 12 shows a conventional overcurrent protection circuit for a step-down type switching power supply apparatus.
In FIG. 12, an input DC power source 201 is formed of a circuit for rectifying and smoothing a commercial power source or a battery. This input DC power source 201 is connected across input terminals 202a and 202b. A current transformer 203 has a primary winding 203a and a secondary winding 203b. One terminal of the primary winding 203a is connected to one (202a) of the input terminals 202a and 202b. The other terminal of the primary winding 203a of the current transformer is connected to one terminal of a switching device 204. The other terminal of the switching device 204 is connected to the cathode of a rectifying diode 205. Furthermore, the other terminal of the switching device 204 is connected to one terminal of an inductance device 206. The switching device 204 connected in this way is configured so as to repeat ON/OFF operation. The anode of the rectifying diode 205 is connected to the other input terminal 202b. 
As shown in FIG. 12, the inductance device 206 and a smoothing capacitor 207 are connected in series, thereby forming a series element. This series element is connected across the rectifying diode 205, thereby forming a smoothing circuit. This smoothing circuit averages a rectangular wave voltage generating across the rectifying diode 205 and obtains a DC voltage.
A voltage averaged by the smoothing capacitor 207 is output across the output terminals 208a and 208b of the conventional overcurrent protection circuit of the switching power supply apparatus shown in FIG. 12. A load 209 is connected across the output terminals 208a and 208b, and consumes the power from the overcurrent protection circuit of the switching power supply apparatus.
A control circuit 210 detects the voltage across the output terminals 208a and 208b, and outputs a control signal for controlling the ON/OFF ratio of the switching device 204 so that a stabilized voltage is output. A first resistor 211 is connected in parallel with the secondary winding 203b of the current transformer 203. In the OFF period of the switching device 204, an exciting current flows through the secondary winding 203b of the current transformer 203, whereby the exciting energy of the current transformer 203 is consumed.
When the switching device 204 is ON, the current flowing through the primary winding 203a of the current transformer 203 is converted into a current corresponding to the winding ratio of the current transformer 203. The converted current flows through a second resistor 213 via a diode 212. Hence, a voltage Vs proportional to the current flowing through the primary winding 203a of the current transformer 203 generates across the second resistor 213.
The voltage Vs generating across the second resistor 213 is compared with the predetermined reference voltage of a reference power source 214 by a comparator 215. When the voltage Vs reaches the reference voltage, the switching device 204 is turned OFF via the control circuit 210. In other words, in the overcurrent protection circuit of the switching power supply apparatus shown in FIG. 12, the current flowing through the switching device 204 is detected in real time. The switching device 204 is controlled so that the instantaneous value of the current does not exceed a certain value. In this overcurrent protection circuit, the current flowing through the switching element 204, an object to be detected, passes through the inductance device 206 and becomes an output current. As a result, the operation for controlling the switching element 204 becomes an operation for restricting the output current.
In the overcurrent protection circuit of the switching power supply apparatus configured as described above, an output current Iout is the average value Iav of a current flowing through the inductance device 206. Furthermore, the peak value of a current flowing through the switching device 204, in other words, the peak value of a current flowing through the inductance device 206 is restricted in real time. The fluctuation width xcex94I of the current flowing through the inductance device 206 is a function of an input voltage Vin and an output voltage Vout, and is given by the following equation (1). In Equation (1), D designates a duty ratio, that is, the ON/OFF ratio of the switching device 204, Ts designates a switching cycle, and Lf designates the inductance value of the inductance device 206.                               Δ          I                =                                                            V                out                            ⁡                              (                                  1                  -                  D                                )                                      ⁢                          T              s                                            L            f                                              (        1        )            
Accordingly, the relationship between the peak value Ip of the current flowing through the inductance device 206 and the average value Iav of the current flowing through the inductance device 206 is represented by the following equation (2).                               I          p                =                                            I              av                        +                                          Δ                ⁢                                  xe2x80x83                                ⁢                I                            2                                =                                    I              av                        +                                                                                V                    out                                    ⁡                                      (                                          1                      -                      D                                        )                                                  ⁢                                  T                  s                                                            2                ⁢                                  L                  f                                                                                        (        2        )            
FIG. 13 is a graph showing current waveforms during the operation of the conventional overcurrent protection circuit. Even when the output current is made constant, the peak voltage differs depending on the input voltage. Hence, in the configuration of the conventional overcurrent protection circuit, control is carried out so that the peak value Ip of the current flowing through the inductance device 206 becomes constant. As a result, the output current Iout has a characteristic of changing together with the fluctuations in the output voltage Vout and the input voltage Vin. FIG. 14 is a waveform graph showing an overcurrent drooping characteristic in the conventional overcurrent protection circuit. When the output voltage Vout lowers as shown in FIG. 14, the output current Iout increases abruptly. In particular, when the inductance value Lf of the inductance device 206 is small, the fluctuation width xcex94I of the current flowing through the inductance device 206 becomes large. The difference between the peak value Ip and the average value Iav of the current increases. As a result, the drooping characteristic becomes worse in this case, and the output current Iout increases. As the output current Iout increases in this way, the currents flowing through the switching device 204 and the rectifying diode 205 become larger. For this reason, devices having a large breakdown resistance are required to be used for the switching device 204 and the rectifying diode 205 of the conventional overcurrent protection circuit. This raises problems of making the circuit expensive and large.
The present invention is intended to solve the problems encountered in the above-mentioned conventional overcurrent protection circuit. More particularly, the present invention is intended to provide a switching power supply apparatus being high in safety and capable of carrying out overcurrent protection stably and securely. This is attained by restricting the output current of a circuit comprising circuit devices having small inductances at a constant value, even when its input voltage and output voltage change.
In order to attain the above-mentioned object, the switching power supply apparatus in accordance with the present invention may comprise:
switching means for forming a rectangular wave voltage by turning ON/OFF an input voltage (Vin),
a smoothing circuit for forming an output voltage (Vout) by smoothing the rectangular wave voltage by using an inductance device and a capacitor, and
overcurrent protection means for restricting an output current by detecting the peak current of a current flowing through the switching means, wherein
in the overcurrent protection means, by using the input voltage (Vin), the output voltage (Vout) and a voltage proportional to the ON/OFF ratio (D) of the switching means, the detected peak current value is corrected by an error signal proportional to the value of (Voutxe2x88x92Dxc3x97Vout).
The switching power supply apparatus in accordance with the present invention configured as described above can restrict the output current to a constant value, even when the input voltage and the output voltage change in a circuit comprising circuit devices having small inductances. As a result, the overcurrent protection circuit can perform overcurrent protection safely and securely.
In addition, the switching power supply apparatus in accordance with the present invention may comprise a multiplier for forming the error signal.
Furthermore, in the switching power supply apparatus in accordance with the present invention, the overcurrent protection means may be further provided with peak voltage holding means.
The switching power supply apparatus in accordance with another aspect of the present invention may comprise:
switching means for forming a rectangular wave voltage by turning ON/OFF an input voltage (Vin),
an insulating transformer having a primary winding connected to the switching means and a secondary winding connected to an output terminal, the winding ratio of the windings being N:1,
output voltage forming means, connected to the secondary winding, for forming an output voltage by carrying out rectification and smoothing by using rectifying means, an inductance device and a capacitor, and
overcurrent protection means for restricting an output current by detecting the peak current of a current flowing through the switching means, wherein
in the overcurrent protection means, by using the input voltage (Vin), the output voltage (Vout) and the ON/OFF ratio (D) of the switching means, the detected peak current value is corrected by an error signal proportional to the value of (Voutxe2x88x92Dxc3x97Vout).
The switching power supply apparatus in accordance with the present invention configured as described above can make the output current constant during the operation of the overcurrent protection means regardless of changes in the input voltage and the output voltage.
Still further, the switching power supply apparatus in accordance with the present invention may comprise a multiplier for forming the error signal.
Still further, in the switching power supply apparatus in accordance with the present invention, the switching power supply apparatus may have an insulating transformer and may be formed of a full-bridge converter.
Still further, the overcurrent protection circuit of the switching power supply apparatus in accordance with the present invention may comprise:
a first series circuit of first switching means and second switching means repeatedly turning ON/OFF alternately and connected in series at a first connection point,
a second series circuit of third switching means and fourth switching means repeatedly turning ON/OFF alternately and connected in series at a second connection point,
the transformer having a primary winding connected between the first connection point and the second connection point,
rectangular wave voltage applying means for applying a rectangular wave voltage to the primary winding of the transformer,
rectifying means for rectifying voltages induced across the secondary windings of the transformer,
a smoothing circuit for smoothing the rectangular wave voltage from the rectifying means by using an inductance device and a capacitor and for outputting a smoothed voltage, and
a circuit for forming a voltage proportional to the output voltage by averaging the voltage at the first connection point.
Still further, the switching power supply apparatus in accordance with the present invention may comprise:
the first series circuit of the first switching means and the second switching means repeatedly turning ON/OFF alternately and connected in series at the first connection point,
the second series circuit of the third switching means and the fourth switching means repeatedly turning ON/OFF alternately and connected in series at the second connection point,
the transformer having the primary winding connected between the first connection point and the second connection point,
the rectangular wave voltage applying means for applying the rectangular wave voltage to the primary winding of the transformer,
a smoothing circuit for smoothing rectangular wave voltages induced across the secondary windings of the transformer by using an inductance device and a capacitor and for outputting a smoothed voltage,
an averaging circuit for forming voltages proportional to the output voltage by averaging the voltage at the first connection point and the voltage at the second connection point, and
an error signal forming circuit for forming an error signal obtained by averaging the difference voltage between the input voltage and the output voltage at the time when the first switching means or the second switching means is ON.
The detected peak current may be corrected by the value of {Dxc3x97(Vinxe2x88x92Vout)} instead of the value of (Voutxe2x88x92Dxc3x97Vout).
Still further, in the switching power supply apparatus in accordance with the present invention, said detected peak current may be corrected by the value of {Dxc3x97(Vinxe2x88x92Nxc3x97Vout)} instead of the value of (Voutxe2x88x92Dxc3x97Vout).
Still further, in the switching power supply apparatus in accordance with the present invention, the detected peak current may be corrected by both a value proportional to (Voutxe2x88x92Dxc3x97Vout) and a value proportional to (Vinxe2x88x92Vout).
Still further, in the switching power supply apparatus in accordance with the present invention, said detected peak current may be corrected by both a value proportional to (Voutxe2x88x92Dxc3x97Vout) and a value proportional to (Vinxe2x88x92Nxc3x97Vout).
Still further, in the switching power supply apparatus in accordance with the present invention, the detected peak current may be corrected by both a value proportional to {Dxc3x97(Vinxe2x88x92Vout)} and a value proportional to (Vinxe2x88x92Vout) instead of the value of (Voutxe2x88x92Dxc3x97Vout).
Still further, in the switching power supply apparatus in accordance with the present invention, said detected peak current may be corrected by both a value proportional to {Dxc3x97(Vinxe2x88x92Nxc3x97Vout)} and a value proportional to (Vinxe2x88x92Nxc3x97Vout) instead of the value of (Voutxe2x88x92Dxc3x97Vout).
The switching power supply apparatus in according with still another aspect of the present invention may comprise:
switching means for forming a rectangular wave voltage by turning ON/OFF an input voltage (Vin),
an inductance device for storing exciting energy at the time when the switching means is ON and the input voltage is applied and for outputting the stored exciting energy at the time when the switching means is OFF, and
overcurrent protection means for restricting the output current by detecting the peak current (Ip) of the current flowing through the switching means, wherein
the overcurrent protection means carries out the arithmetic operation of {(1xe2x88x92D)xc3x97(Ip+Kxc3x97Vout)} with respect to the detected peak current Ip and with K used as a constant, and determines the ON period of the switching means so that the arithmetically operated value becomes constant.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.